The invention is related to a distance holder for connection to, and rotation, with a drill string in an earth formation drilling device arranged to supply a jet of abrasive fluid for the purpose of providing a borehole by removing earth formation material through abrasion, where the distance holder comprises a chamber that is essentially rotational symmetric and which faces the earth formation material, and a jet nozzle arranged for discharging a jet of the abrasive fluid in said chamber.
Such a distance holder is disclosed in WO-A-2005/040546. Said prior art distance holder provides an abrasive fluid jet which is directed towards a slot in the circumference of the chamber. The jet, which is directed through the slot, exerts an abrasive action on the earth formation within the chamber whereby a cone shaped bottom is obtained. Subsequently, the jet direction is reversed by the lowest part of the bottom into an upward direction. The cuttings or abraded particles as well as the abrasive particles are transported to the surface by the fluid; at some height above the bottom the abrasive particles are extracted from the fluid and fed back into the jet nozzle. By means of the fluid that is jetted through the nozzle, the abrasive particles enter a new cycle of abrasive action, and so on.
In practice it appears that the wall of the hole thus obtained lacks a certain smoothness. A good borehole quality is however important for obtaining earth formation data by means of sensors. Pad-type down-hole evaluation sensors are applied onto the wall of the borehole, and the contact between such sensors and said wall is gravely impaired by a less than smooth borehole wall quality. Moreover, parasitic pressure losses may occur, and furthermore borehole cleaning by the fluid flow through the annulus towards the surface may be impaired. Also, energy is lost when forming grooves in the rough borehole wall.